Loss of p53 function is the most common genetic change in human malignancy. There is evidence that p53 inactivation may be involved in cell immortalization. Induced p53 expression in p53 negative EJ bladder tumor cell causes permanent growth arrest/senescence. Thus, senescence appears to represent an important cell program that may require p53 inactivation for tumor evolution. We have shown that this terminal differentiation program can be synchronously induced in response to p53 over-expression in some cell contexts. We have further identified a novel p53 signaling response involving sustained MAPK activation, and generated preliminary evidence that this response contributes to the growth arrest phenotype. This proposal is specifically directed at elucidating the role of MAPK activation in p53 induced growth arrest/senescence, the mechanisms responsible and the effector pathways involved. In Aim 1, we plan to correlate MAPK activation with p53 induced growth arrest in different cell types, investigate the contribution of activated MAPK to permanent growth arrest of tumor cells using tet regulatable expression, and genetically dissect the role of MAPK in response to DNA damaging agents. In Aim 2, we will investigate the biochemical pathways by which p53 activates the MAPK cascade including investigations of possible autocrine growth factors. In Aim 3, we will investigate MAPK effector pathways and their functions in growth arrest utilizing genetic approaches as well as establish animal models involving wt p53 or activated MEK transgenes. These studies should make it possible to investigate in vivo correlates of the in vitro growth arrest/senescence response induced by these genes. The long term goals of this project are to understand how MAPK activation contributes to p53 induced permanent growth arrest/senescence as a means of developing novel approaches to therapy by targeting this terminal differentiation program in tumor cells.